Burner control system



Feb. 11, 1941. w. H. GILLE BURNER CONTROL SYSTEM Filed Sept. 30, 1958 2 Sheets-Sheet l Qmacntor Willis HQ Gille WW M attorney .Fig,2

Patented Feb. 11, 1941 PATENT OFFICE 2,231,420 BURNER CONTROL SYSTEM Willis H. Gille, St. Paul, Minn, assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn, a corporation of Delaware Application September 30, 1938, Serial No. 232,607

14 Claims.

The present invention relates to a burner control system and more particularly to one having improved means for checking the presence of a pilot flame before fuel is supplied to the main 5 burner.

It is the conventional practice with burner control systems to provide some means for insuring that fuel is not supplied to the main burner until such time as the pilot burner is properly ignited. In relatively small burner installations the speed of response of the pilot flame responsive means is not particularly important so that it is possible to use various temperature responsive devices for performing this function. In large industrial installations, however, it is highly imperative that particularly in the event of flame extinguishment, the supply of fuel to the main burner be interrupted immediately. Many of these large burners consume an enormous quantity of fuel and within a very short period of time sufllcient winignited fuel may be supplied to the fuel chamer to cause a serious explosion. It has, accordlngly, been proposed to place an electrode in the pilot ner flame, the burner flame forming a portionv if a conductive path. The potential across ine burner flame is then used' to control the fuel flow to the main burner. In one preferred form, the potential across the pilot burner flame is used to control the grid volt e of a space discharge tube, the output of which controls a relay which, in turn, controls some fuel flow controlling device such as a gas valve or an oil burner motor.

The difficulty that has arisen with devices of the type last mentioned is that it is often necessary to have a lead of appreciable length between the casing housing the space discharge tube relays and other associated control devices and the electrode positioned in the pilot burner flame.

burner be connected to a voltage source so that a relatively high potential exists between the pilot burner and the electrode. In order to have the apparatus safe, it is necessary to ground the pilot burner and to shield the lead between the electrode and the control apparatus, this shield being also grounded. The presence of this long grounded lead between the'oontrol apparatus and the electrode creates a possible hazard of a false indication of a flame by reason of leakage or capacity effect between the lead and the shield surrounding the same. As will be noted from the In order to raise the grid voltage upon the presence of a flame, it is necessary that the pilot above description, the insulation between the lead or conductor and the surroundingshield is, in effect,v connected in parallel with the gap between the pilot flame electrode and the pilot burner, both being subjected to quite a high voltage. Where an alternating voltage is employed as is often desirable, the capacity effect in a long lead may be very appreciable and in view of this high voltage it is very possible for the current flow which is normally indicative of the presence of a pilot burner flame to actually take place between 10 the conductor and the surrounding shield. This danger has previously made it desirable to employ direct current voltages with the resultant increase in expense in order to get satisfactory operation. Even-then the danger of leakage was always present to a certain extent.

A further difficulty with these prior devices is the possibility of a false indication of flame being given as the result of a grounded electrode or similar condition causing a very low resistance between the electrode and ground. Inmy prior application, Serial No. 61,398, filed January 2, 1936, I have disclosed such a system in which a check is provided against a grounded electrode before the pilot burner is placed in operation. It often happens, however, that such a grounded condition may not arise until after the main burner has been supplied with fuel. As many burners will not stay properly ignited without the presence of an ignited pilot burner, it is desirable that there be some means of terminating burner operation if the resistance between the electrode and ground becomes abnormally low at any time.

While a system has been devised overcoming certain of the above named disadvantages, this system provides the disadvantage that its operation demands the decrease of the output current of a space discharge triode unit upon the presence of flame and the increase of such output current in the event of there being no flame. This has the obvious disadvantage that any failure of this trlode will give a false indication of flame.

An object of the present invention is to provide a burner control system having a flame de ecting arrangement of the type referred to above in which the effect of capacity and leakage is minimized and in which upon the existence of excessive capacitive effect or leakage, the grid of the space discharge tube having the flame as a portion of its grid circuit is subjected to such a large negative voltage that the discharge of the tube and consequently the flow of fuel to the burner is prevented.

A further object of the present invention is to 'perative that it be assured that the burner provide an intermediate shield between the outer rounded shield and the conductorwhich is connected to thegrid in such a manner that normal capacitance and leakage do not afl'ect the operation of the system and that in the event of excessive capacitive efiect or leakage the grid is subjected to a negative voltage uch that the flow of fuel to the burner is prevented.

A further object of the present invention is to provide a burner control system having such a flame detecting arrangement in which th presence of an abnormally low resistance between the flame electrode and ground will always prevent further burner operation by increasing the negative bias on the grid.

A further object of the invention is to provide such a burner control system in which upon either the flame being extinguished or the resistance between the flame electrode and ground being reduced so as to make liable a false indication of flame, the negative bias on the grid "circuit is increased so as to prevent burner operation. I

Further objects of the present invention will be apparent from a consideration of the accompanying specification, claims and drawings of which:

Figure 1 is a schematic view of one form of the improved burner control system:

Figure 2 is a diagrammatic view of a portion of the system of Figure 1;

Figure 3 is a schematic view of a modified form of the improved burner control system; and of which Figure 4 is a vector diagram showing the relation. between the various voltages across diiierent points of the circuit of Figure 3.

Referring to the drawings for a more complete understanding of the present invention, the system is shown as employed in connection with an oil burner. It is to be understood, however, that the invention is not so limited and that any other fuel burner arrangement such as a gas burner and an associated gas valve may also be employed. The oil burner is indicated in the drawing by the reference numeral ID. The burner is of conventional form and comprises a nozzle H and a motor |2 which serves to drive a blower forcing an atomized mixture of fuel and air through the nozzle I. Located adjacent to the burner 55 is a pilot burner |4 supplied by a pipe L? with a suitable fuel such as gas.

It is to he understood that the fuel issuing from nozzle H is ignited by the burner l4 if the same is properly lighted. It is, accordingly, im-

4 is lighted before the oil burner is placed in operation. While the pilot burner is shown as of the constantly burning type, it is tobe understood that the invention is equally applicable to one in which the pilot burner is lighted by separate ignition means each time the oil burner is placed into operation.

A relay is employed to control the operation of .the oil burner. This relay is indicated by the reference numeral 20 and comprises a, relay coil 2| and a pair of switch blades 22 and 23.

Switch blade 22 is biased into engagement with a fixed contact 24. The switch blade 23 is biased out of engagement with a fixed contact 25. Upon energization of the relay coil 2|, switch blade 22 is moved out of engagement with contact 24 and switch blade 23 into engagement with contact 25. A condenser 21 is connected inparallel with relay coil 2| to con- 'Willbe tering action of the relay. A space discharge device 30 controls the energization of relay 20. The space discharge device 30 may take the form of an ordinary thermionic vacuum tube although the invention is not so limited. In any event, the device comprises a cathode 3|, a grid 32, and an anode 33.

A combination step-up step-down transformer is employed for supplying the necessary power for operation of the vacuum tube and the control system associated therewith. This transformer is indicated by the reference numeral 35. The transformer comprises a line voltage primary 31 connected to line wires 38 and 39 leading to a suitable source of power (not shown). The transformer also comprises a center tapped secondary 4| for energizing the heater element of the cathode 3|. In the embodiment shown, the cathode 3| is of the filament type although it is to be understood that an indirectly heated cathode may be employed. The transformer also comprises a secondary 42 which is for the purpose of supplying the necessary voltage vfor the anode circuit of the vacuum tube and the various voltages applied to the grid circuit.

A thermostat 45 is used for controlling the energization of the filament and consequently the operation of the vacuum tube. The thermostat 45 comprises a bimetallic element 46 to which is secured a contact arm 41 adapted to be moved in and out of engagement with a fixed contact 48. A magnet 43 is associated with the contact arm 41 and serves to impart a snap action thereto. This snap action movement which the magnet causes both reduces sparking in the contacts to a minimum and also insures the necessity of a temperature differential in the operation of the thermostat to prevent an excessive number of operations or a chattering action. The thermostat 45 may be located at any desired controlling point, such as a room or other space beof illustration, the switch comprises a pair of switch blades 5| and 52, the lower end of switch blade 52 being pivotally mounted as at 53.- Located underneath the right-hand end of switch blade 52 is a bimetallic element 54. The bimetallic element 54 is shown in its cold position, in which position it is effective to maintain switch blade 52 in engagement with switch blade 5|. Located adjacent to bimetallic element 54 is a heater' element 55. This heater element when energized serves to warp bimetal element 54 to the right. If this heating action is continued sufliciently long, the bimetal element 54 warped sufiiciently to the right to permit switch blade 52 to drop away from switch blade 5|. After this has happened, it is impossible for the switch blades 5| and 52 to move together without being reclosed manually.

The grid 32 is connected through conductors 60 and 5|, resistance 62, and conductor 63 to the uppermost terminal of secondary 42. Inasmuch as the opposite end of the secondary 42 is the one which is connected to the anode 33,

the upper end of the secondary 42 is the point of lowest potential when the direction of the potential is such that the tube 38 passes current. The mid point of filament secondary 4| is connected by a conductor 55 to a tap 51 of the secondary 42. It will be noted that the tap 81 is at an intermediate point of the secondary 42. Thus considering the half cycle during which the tube 30 is passing current, which half cycle is the only one of interest so far as the grid circuit is concerned, the connections between the grid 32 and the secondary 42 through resistor 82 tend to cause the grid to assume a potential below that of the cathode 3|. If it were not for the potential drop through the resistor 52, this diflerence in potential would be the potential difference between tap 61 and the upper end of the secondary 42. However, due to the potential drop through resistor 62 this potential difference between the grid and the cathode is considerably reduced but is still sufficiently great that when the voltage on grid 32 is determined solely by this connection, the tube is biased to such a point that no anode current flows therethrough.

" It will be noted that the lowermost point of the secondary 42 is connected to ground at 58. This point is the point of highest potential during the half cycle that the tube is passing current. A second circuit exists between the grid 32 and the secondary 42, when the pilot burner is lighted, as follows: grid 32 through conductors 60 and I0, electrode pilot burner flame 12, pilot burner |4, ground connection I3, through the ground connection 88 and conductor 14 to the lowermost point of secondary 42. As soon as this connection is established, the potential of the grid 32 is raised sufliciently as to cause tube 38 to pass anode current.

Surrounding the conductor 18 is a shield 15. While this shield is shown as connected over only a portion of the length of conductor 19, it is to be understood that in actual practice the shield 15 will extend all the way from the electrode to the point where the conductor '||l is entirely enclosed in a housing for the control apparatus. In order to insure that the shield I will at no time be at a dangerous potential with respect to ground, the shield is connected to ground as at 16. Interposed between the shield and conductor 18 is an intermediate shield 18. The conductor 10 and shield I8 are separated from each other by an insulating layer 82 of extremely high dielectric strength. A similar layer 83 insulates the shield 18 from the outer shield 15. Shield 18 is connected through a conductor I9 to a tap 88 which during the conductive half cycle is at a lower potential than the tap 81 to which the cathode 3| is connected. The purpose of this intermediate shield and connection I9 will be discussed in more detail later.

Operation of species of Figures 1 and 2 The various elements of the system are shown in the position which they occupy when the temperature to which thermostat 45 is responsive is through conductor 85, switch blades 52 and 5|,

conductor 86, contact 48, switch blade 41, bimetallic element 48, conductors 81 and 88, filament 8|, and conductors 89 and "to the lower terminal of secondary 4|. The establishment of this circuit will start the heating of the filament 3|.' At the same time as the circuit is established to the filament 3|, the following circuit is established to the heating element 55 of the thermal safety switch 59: from the upper terminal of secondary 4| through conductor 85, switch blades 52 and 5|, conductor 86, contact 48, contact blade 41, bimetal element 46, conductors 81 and 9|, heating element 55, conductor 92, switch blade 22, contact 24 and conductors 94 and 80 to the lower terminal of secondary 4|.

As soon as the filament 3| becomes suillciently heated, current will flow through the following anode circuit providing that the grid potential is sulflciently high: from the lowermost point of secondary 42 through conductors 14, 95 and 29, relay coil 2|, conductors 28 and 91, anode 33, cathode 3|, secondary 4|, and conductors 65 to the tap 51 of secondary 42. Unless the pilot burner is ignited; the grid will not have a potential sufficiently high to permit flow of current through this circuit. So long as there is no flame 12, the grid 32 is connected only to the upper end of the secondary 42, which during the conductive half cycle is at the lowest potential. If there is a flame 12, however, the grid is not only con-' ductor Hi9, burner motor I2, and conductor |0| to the other line wire 38. The establishment of this circuit results in the burner motor forcing a mixture of fuel and air through the nozzle H, which mixture is ignited by the pilot burner l4. In the case of a gas burning system, the circuit just traced would be to the gas valve so that the establishment thereof causes the valve to open, admitting gas to the burner.

Energization of relay 2| also results in switch blade 22 being moved out of engagement with contact 24 with the result that the previously traced circuit through heating element 55 is interrupted. The result of this is that the warping of bimetallic element 54 is terminated. If the energization of relay 2| occurs suiiiciently quickly, switches 5| and 52 will accordingly never be separated. If, however, the pilot burner is extinguished at the time that the thermostatcalls for heat or if for some reason the control apparatus is faulty, the heating of the bimetal will continue sufliciently long to allow switch blade 52 to separate from switch blade 5|, completely deenergizing the filament circuit and consequently preventing the operation of the apparatus.

Inasmuch as the flame I2 has an extremely high resistance, it is necessary that the voltagev between-grid 32 and the burner H be relatively high in order that the establishment of the flame shield corresponding to shield 15. It was necessary that this shield be grounded in order to eliminate any danger of a person accidentally engaging the conductor receiving anelectrical shock. The presence of this grounded shield,

however, resulted in a path between theconductor II and ground parallel with the path to the In the prior art systemsthe 55 power, any slight conductance that may take flame. It can be readily seen that particularly where an alternating current voltage was employed and there was appreciable capacity effect, the total impedance between conductor III and the shield 15 might be very little higher, if any, than the impedance of the flame I2. Even disregarding the capacity effect, any very slight tendency of leakage through the insulation between the conductor and the shield might result in a false indication of the flame. Such a false indication would, of course, be highly undesirable since the user would be relying upon the control apparatus for insuring that the burner was not turned on if there was no pilot fiame.

The presence of the intermediate shield I8 in the device of the present invention not only lessens the tendency for capacity or a slight amount of leakage to effect the operation of the system but also insures that if such leakage or capacity efiect become excessive the system will prevent fuel being supplied to the burner, or in other words, will fail safe." The function of this intermediate shield can perhaps best be understood by reference to the schematic diagram of Figure 2. In this diagram, a number of elements of the system have been left out and the rest of the elements have been rearranged to more clearly indicate the relative potentials which the various elements are subjected to. In order, moreover, to more clearly understand the relative potentials of the various elements, the various potentials are indicated on the drawing at the values they may assume during the conductive half cycle. Thus let it be assumed that the cathode is at zero potential and the lower end of the secondary 42 is at plus 250 volts and the upper end at minus 150 volts. By reason of the potential drop through resistor 62, the grid 32, in the absence of flame 12, is at a potential close to [that of tap 80, or, in other words, sufficiently negative with respect to the cathode that no anode current passes through the tube 30. Under these conditions, if there were no intermediate shield I8, the potential difference between the conductor I and the outer shield I would be over 250 volts. This relatively large potential difference would be sufiicient to make the effects of any slight leakage or capacitance very evident and might lead to a false indication of the presence of flame I2. By reason, however, of the intermediate shield I8 connected to the winding 42 at tap 80 there is practically no potential difference between the. conductor III and the shield I8. Thus even though there is a slight amount of leakage, the potential difference between conductor II and the intermediate shield I8 is so slight that the leakage and capacitance will have practically no effect on the potential of the grid. While the grounded shield I5 still exists, the shields I8 and I5 are connected directly across the transformer secondary 42. Inasmuch as the transformer secondary 42 is capable of delivering a substantial amount of place between shields I8 and I5 will not change the potential difference between these two shields. In other words, the shield I8 will continue to remain at the potential of tap 80. Even after the 70 grid 32 is connected through the flame I2 with the lower or high potential end of secondary 42, the voltage between conductor I3 and the intermediate shield I8 will still be relatively small. It

must be borne in mind that all that it is neces-' sary to'do is to raise the grid potential slightly capacity effect and even though there is a slight by an amount less than 80 and 61.

If for some reason or other, there should be an excessively low impedance between conductor III and the intermediate shield I8, the most serious condition that could happen would be that the grid 32 would assume a potential substantially equal to that at tap BII. This potential, as previously indicated, is such that the grid, if subjected to this potential, will prevent any flow of current through the tube. In other words, the tap 80 is at the cut-off" potentiaL- With the species just described, it is thus possible to employ alternating current exclusively for the operation of the system and avoid the disadvantages of previous systems of this type. It will further be seen that the defects of the prior systems have been overcome in an extremely simple manner.

Species of Figures 3 and 4 While the system of Figures-1 and 2 has a distinct advantage over certain prior systems, this system does have the disadvantage, however, that it is not able to differentiate between a low resistance condition caused by a flame and that caused by a short circuit between the electrode and the voltage between taps burner. As pointed out previously, it is desirable that the apparatus shut down in the event of a short circuit or partial short circuit condition between the burner electrode and the burner. If such shut-down does not occur, a false indication of flame will be given and the apparatus will fail to perform the function for which it was intended. The system of Figures 3 and 4 avoids 3 a fixed contact I I 4 and is adapted to be moved out of engagement therewith upon the energiza- 'tion of relay coil III. Switch blade H3 is biased out of engagement with a fixed contact I I5 and is adapted to be moved into engagement therewith upon energization of relay coil I I I. The switch blade H3 and contact H5 connect to conductors II! and H3, respectively, leading to a burner, valve, or other heater controlling device. A condenser I 23 is connected in parallel with relay coil III to filter out the pulsations in the current supplied thereto and thus eliminate chattering of the relay.

The energization of relay coil H0 is controlled the reference numeral I23. This tube is preferably a vacuum type although other suitable tubes such as gas filled electronic tubes may be employed. The tube I23 comprises a cathode I24, a heater I25 therefor, a grid I26, and a plate I21. The construction being entirely conventional, no further explanation is needed.

A combination step-up step-down transformer I supplies power for operation of the system. The transformer comprises a primary winding I3I connected to line wires I32 and I33 leading to a suitable source of power. The transformer also comprises two secondary windings I35 and I36. The secondary winding I36 is employed for energization of the cathode heater I25. The secondary I35 is employed for supplying the power equivalent to the switch of Figure 1.

for the control and output circuits of vacuum tube I23.

The operation of the system is controlled by a room thermostat I31 which is of conventional type and comprises a bimetallic element I38 to which is secured a contact arm I39. The contact arm.I39 is adapted to be moved into engagement with a fixed contact I40 upon a temperature fall. A magnet MI is associated with the contact blade I39 and serves to impart a snap action thereto.

The thermostat I31 controls the operation of the system, as in the species of Figure l, by controlling the energization of the cathode heater. Also controlling the energization of this heater is a thermal safety switch I43 which is This switch comprises a rigid switch blade I44 and a pivoted switch blade I45, the pivoted switch blade being held in the position shown in the drawing by a bimetallic element I46 which is heated by a heater I41. The operation being the same as switch 50 of Figure I, no further explanation is deemed necessary.

The pilot burner is designated in Figure 3 by the reference numeral I50. This pilot burner is connected to ground by ground connection I5I. An electrode I52 is adapted to project into the burner flame, this electrode being connected to the grid I26 by a conductor I53. Surrounding the conductor I53 is a double shield consisting of an inner shield I54 and an outer shield I55. It is to be understood that this shield corresponds in structure to the shield of Figure 1 and that the two shield members are insulated from each other and from the conductor I53 by a suitable dielectric.

For convenience in the subsequent description, the various taps of the transformer secondary I85 are designated by the reference numerals I60, I6I, I62, I63 and I64. A conductor I66 extends from the tap I6I to a resistor I61. A conductor I68 extends from the tap I64 to a condenser I69. A conductor I10 is connected at one end to the resistor I61 and'at its other end to'a conductor I1I extending from condenser I69. The junction of conductors I10 and I" is designated by the reference numeral I12. The tap I63 is half way between the taps I6I and I64. The tap I63 is connected by means of a conductor I14 to the center tap I15 of an autotransformer winding I16. The other end of the right half or primary half of the autotransformer is connected to Iunction I12. The opposite end of the winding I16 of the autotransformer is connected to ground by a ground connection I11. As will be more apparent from the subsequent description of the operation, the portion of secondary winding I35 between taps I6I and I64, the resistor I61, and condenser I63 constitute a phase shifting bridge between the output terminals I63 and I12 of which the primary portion of the autotransformer winding I16 is connected. The impedance of resistor I61 is preferably the same as that of condenser I69 so that there is impressed across the primary portion of the winding of secondary I16 a voltage which is QO-degrees'displaced in phase from the voltage of the winding I35. 1

Connected to the terminal I12 of the autotransformer winding'l16 through conductors I80 and I8I is a condenser I82, which condenser is in turn connected through conductors I83 and I53 with the flame electrode I52, the Junction of conductors I83and I53 being indicated by the,

reference numeral I84. The flame electrode I62 is in turn connected to the burner flame, burner I50, ground connections I5I and I11 with the other terminal of the autotransformer windin I16. The autotransformer I16, the condenser first phase shifting bridge.

It will be noted that the grid I26 is connected at junction I84 to conductor I53 which is one of the conductors joining condenser I62 to electrode I52. The junction between condensers I82 and flame electrode I52 constitutes one of the output terminals of the second phase shifting bridge. The other output terminal of the second phase shifting bridge is the center tap I15 of autotransformer winding I16. The cathode I24 is connected to this tap I15 through conductor I85, a portion of the transformer winding I35 between taps I62 and I63, and conductor I14. It will thus be noted that the grid circuit of tube I23 includes in series the output of the second phase shifting bridge and the section of the transformer secondary between taps I62 and I63. As will be pointed out later, the anode I21 is connected to the tap I so that during the half cycle of the system in which the vacuum tube is passing current, tap I60 is the point of highest potential of secondary I35. Consequently, duringthis half cycle'the tap I62 is at a higher potential than tap I63 so that the inclusion of this portion of the transformer secondary in the connection between the cathode and one terminal of the second phase shifting bridge results in the grid being biased negatively in the event of the phase shifting bridge having a zero output or an output which is 90 degrees displaced in phase to the voltage of secondary I35. The value of the biasing voltage between taps I62 and I63 is so chosen that it is effective to bias the grid so far negatively that insufficient current flows through relay coil II I to operate the same. Operation of species of Figures 3 and 4 It will be assumed that the temperature in the space in which thermostat I31 is located falls sufficiently so as to cause switch arm I39 to engage contact I40. When this takes place, a. circuit will be established to filament winding I25 as follows: from the lower terminal of secondary I36 through conductor I81, bimetal I38, contact arm I39, contact I40, conductors I68 and I89, switch blades I45 and I44, conductor I90, cathode heater I25, and conductors 'I9I and I92 to the other terminal of secondary I36. This will result in the cathode heater I25 warming up to heat the cathode I24 so that the tube becomes conductive providing the other conditions necessary to the conductance of the tube are established.

It will be noted that this filament circuit includes switch blades I44 and I45 of the thermal safety switch. At the same time that the circuit to filament heater I25 is established, a circuit is also established to the safety switch heater I41 as follows: from the lower terminal of secondary I36 through conductor I81, bimetal I38, contact arm I39, contact I40, conductors I88 and I93, heater I41, conductor I94, contact II4, switch blade H2, and conductors I95 and I92 to the impressed across it the output voltage of the upper terminal of secondary I36. The result is 1 'to being in phase that the bimetallic element is heated by heating element I41 so that unless heater I4! is deenergized within a predetermined period of time, switch blades IM'and I 45. will be separated to deenergize the system.

The conductivity of the tube depends upon the negative bias of the grid being suiilciently small or, in other words, 'upon the potential of the grid being sufficiently high with respect to that of the cathode. As pointed out previously, the potential of the grid is determined by the phase relation of the output voltage of the second of the two cascaded phase shifting bridges. The phase of the output of this second bridge depends upon the conductivity of the flame.

In order to enable a clearer understanding of these two phase shifting bridges, their relation to each other and the efiect of a change in flame resistance, a vector diagram has been prepared in Figure 4 wherein there is shown the relationship between the various voltages across the different points of the phase shifting circuit. Referring to Figure 4, it will be noted that there is a long vertical vector pointed upwardly. This vector represents the voltage of the entire transformer secondary I35. Superimposed upon this long vector is a shorter vector, the extremity of which is determined by the arrow ISI. This shorter vector represents the voltage between taps I64 and Hit. The voltages across resistance I67 and condenser E69 are vectorially represented in Figure 4 by the vectors i'l2--E6I and [Bl-I12, respectively. In order to more clearly identify these two vectors with Figure 3, the resistance I 67 and condenser I 63 have been superimposed in dotted lines upon the corresponding vectors. The resultant output voltage of the first phase shifting bridge is designated by the vector i63I'I2, this voltage being the voltage which is impressed across the right-hand primary half of the autotransformer winding I716. This voltage I63--I'IZ, by reason of the fact that the resistance E61 and condenser I 69 are chosen to have equal impedances, is displaced 90 degrees in phase from the voltage 564-469 of the secondary 35. Due to the autotransformer efi'ect, there will be induced in the left-hand side of winding I16 a voltage represented by the vector I'll-463. There is thus produced across the autotransformer a voltage ill-H2 which is 90 degrees displaced with respect to the voltage i64i6|.

The voltages across the flame and across condenser l 82 are designated by the vectors I-11--I 8t and lat-H2, respectively. Again, the condenser l-SZ has been indicated in dotted lines to aid in the identification of the vector. Similarly, on the vector ill-Mt, the flame resistance has been indicated in dotted lines as a variable resistor,

The vector diagram is shown with the vectors in the position assumed when a substantial normal flame exists. Under these conditions, the voltage between junctions I 6.3 and P84, represented by the vector I 63-I84, is nearly 90 degrees displaced from the voltage ITI-I-IZ so that it is almost in phase with the original supply voltage l6 lI 5i. At least it is suiflciently close therewith that a substantial component thereof is in phase with the supply or plate voltage. This component being in phase and having the positive end thereof connected to the grid I 26 resultsin the voltage |63!l84 op posing the voltage I-63--4I62 tending to drive the grid negatively with respect to the cathode. The

, biasing voltage is designated in the vector dia- Ihe. vector ramby the vector I63- I 5-2.

IG2I84 represents the resultant voltage applied to the grid and the vector ISL-I84 the component thereof in phase with the plate voltage. The result isthat when a normal flame exists so that the vector diagram of Figure 4 is applicable, the grid biasing efiect of the voltage I$*3--I62 is sumciently overcome to permit the tube to pass current as soon as the cathode becomes heated. Upon this happening, a circuit is established through relay III as follows: from the positive terminal of secondary I0 through conductor I98, relay coil II I, conductors I99 and 200, anode I21, cathode I24, and conductor :I85 to tap I62.

The energization of relay coil I II as a result of the establishment of this circuit will cause switch blade 2 to be moved out of engagement with contact M4 and switch blade II3 into engagement with contact H5. The movement of switch blade M2 out of engagement with contact III terminates the energization of the thermal safety switch heater I41. The movement of switch blade I I3 into engagement with contact H5 results in the burner, valve, or other similar device being energized to cause fuel to flow to the burner.

The condition has been described in which the burner I50 is ignited. If the burner is not ignited, the impedance of the flame gap is extremely high. The result is that a. very large portion of the voltage across the autotransformer exists across this gap. The effect of this is to rotate the voltage vector IB3'I'84 in a clockwise direction until'this vector is nearly in phase with the voltage Ill-J12 or degrees displaced from vector I=64I EI. Under these conditions, the voltage I63I'84,being 90 degrees displaced from the biasing voltage I63-4I82 and from the plate voltage I63I60, is ineffective to overcome this biasing voltage. The result is that the grid is biased negatively sufiiciently to prevent any appreciable current flow to relay I I I.

If on the other hand, a short circuit condition exists between the electrode and the burner I50, only a small portion of the voltage across the autotransformer exists across the flame gap, the majority of this voltage being across the condenser. I'ne result is that the voltage vector I6-3-I8-4 is rotated in a counter-clockwise direc tion from the position occupied in Figure 4 until this voltage vector is nearly degrees displaced in phase from the voltage vector III I'I2 or 90 degrees displaced from the supply voltage 6'4I-6I. The result is that again the voltage I'84-i63 is unable to counteract the effect of the biasing voltage I-6-3--I62. The result is that the grid is again biased negatively to a point where insufllcient current flows through relay coil III to operate the same.

It will be noted from the preceding description that the relay is not operated if the flame resistance is either abnormally high or abnormally low. In this way, it is assured that there will be no danger of a false indication of flame by reason of a. short circuit condition. This is accomplished, moreover, without the use of any triode depending upon a decrease in output. current .upon the establishment of flame. Thus if, with the system of the present invention, the triode should fail, the relay will go to its deenerg-ized position. In this connection, it is to be understood that the expression relay or relay meansflas used in this description and in the appended claims, is intended to be broad enough to include other electronic devices which in themselves operate as relays.

III)

ass-taco It will be noted that the present species also includes a pair of concentric shields surrounding the conductor I53 between the grid and the flame electrode I52. As in the other species, the outer shield I55 is grounded at 202. The inner shield I54 is connected by conductors 203 and I80 to the junction I12. If for any reason the impedance between the inner shield I54 and conductor I53 should become sufficiently low as to interfere with the proper operation of the system, the system will be made incapable of energizing the relay, as in the previous species. It will be noted that as this impedance between the conductor I53 and shield I54 becomes reduced sufliciently, the grid I26 tends to assume the potential of junction I12, so that the second phase shifting bridge has no effect upon the voltage between grid I26 and tap I63. The result is that a voltage between grid I26 and cathode I24 consists of two components, one the phase voltage |53-I62 and the voltage 53-412 which is 90 degrees displaced from the supply voltage. The voltage ISL-I62 which biases the grid below the cut-off point is thus unopposed with the result that the relay I I I is not energized.

Thus with this species as with the previous one, it is assured that the benefits of a shielded electrode lead are obtained without any danger of false indication of flame by reason of excessive leakage or capacitive effect between the electrode lead and its shield.

While I have described certain specific embodiments of my invention for purposes of illustration it is to be understood that I am to be limited only by the scope of the appended claims.

I claim as my invention:

1. In a flame detaching system, a burner, relay means, a grid controlled space discharge device comprising an anode, a cathode, and a control grid, means for operatively connecting said device to said relay means so that said relay means is operated only when the current flow between said anode and cathode is above a predetermined value, a periodically varying source of voltage, a control grid circuit comprising in part a connection between said grid and said source of voltage through a conductor leading from said grid and connected to an electrode extending into the burner flame, the burner being connected to said source of voltage through ground at a potential sufficiently high that when said grid is connected thereto the anode current is above said predetermined value, a shield surrounding the greater portion of said conductor and also connected to ground, and an intermediate shield coextensive with said first mentioned shield between said conductor and said first mentioned shield, said intermediate shield being connected to said source of voltage at a point that is sufllciently negative with respect to the point at which the cathode is connected that in the evenlt of excessive capacitive effect or leakage through the insulation surrounding said conductor said grid will be subjected to a voltage such that the current flow between said anode and cathode is insufiicient to operate said relay means.

2. In a flame detecting system, a burner, said burner being electrically connected to ground, relay means, a space discharge device comprising an anode, a cathode, and a control grid, a periodically varying source of voltage, connections between said source of voltage, said anode, said cathode, and said relay means to form an output circuit for said device, a connection between a portion of said circuit and ground, said space point of connection being so located that when discharge device is passing current the highest potential is at ground potentia nection between said grid and ground a conductor extending between said grid electrode separated from said grounded o by a gap adapted to be bridged by the but flame, a shield surrounding the greater portion of said conductor and connected to ground, and means for preventing a false indication of the presence of a burner flame by reason of excessive capacitance or leakage at any point through the insulation surrounding said conductor.

3. In a flame detecting system, relay means, a burner, said burner being electrically connected to ground, a space discharge device comprising an anode, a cathode, and a control grid, a -periodically varying source of voltage, connections between said source of voltage, said anode, said cathode, and said relay means to form an output circuit for said device, a connection between a portion of said circuit and ground, said connection being so located that when said space discharge device is passing current the point of highest potential is at ground potential, a connection between said grid and ground including a conductor extending between said grid and an electrode separated from said grounded burner by a gap adapted to be bridged by the burner flame, a shield surrounding the greater portion of said conductor and connected to ground, and an intermediate shield between said conductor and said shield and coextensive with said first named shield, said intermediate shield being connected to said source of voltage at a point such that in the event of excessive capacitive effect or leakage through the insulation surrounding said conductor, said grid will be subjected to a voltage sufficiently negative with respect to the cathode that the current flow between said anode and cathode is insufllcient to operate said relay means.

4. In a burner control system, a main burner, a pilot burner for igniting said main burner, a space discharge device comprising an anode, a cathode, and a control grid, current responsive means for causing a flow of fuel to said main burner when the current flow between said anode and cathode is above a predetermined value, a source of voltage, connections between said anode, said cathode, and said source of voltage including said current responsive means, a first connection between said grid and said source of voltage at a point operatively negative with respect to the cathode, a second connection between said grid and said source of voltage comprising a conductor leading from said grid to an electrode extending into the pilot burner flame and connections through ground between said pilot burner and said source of power at a point operatively positive with respect to the cathode, a grounded shield surrounding the greater portion of said conductor, and means for preventing a false indication of the presence of a pilot burner flame by reason of excessive capacitance or leakage at any point between the conductor and the shield.

5. In a flame detecting system, a burner, relay means, a space discharge device comprising an anode, a cathode, and a control grid, means operatively connecting said device to said relay means so that said relay means is operated only when the current flow between said anode and cathode is above a predetermined value, a source or voltage, connections between said anode, said cathode, and said source of voltage including said relay means, a first connection between said grid and said source of voltage at a point operatively negative with respect to the cathode, a second connection between said grid and said source of voltage comprising a conductor leading from said grid to an electrode extending into the burner flame and connections through ground between said burner and said source'oi power at a point operatively positive with respect to the cathode, a grounded shield surrounding the greater portion of said conductor, and means for preventing a false indication of the presence of a burner flame by reason of excessive capacitance or. leakage between the conductor and the shield, said last named means comprising an intermediate shield coextensive with said first named shield and connected to said source of power at a point operatively negative with respect to the cathode.

6. In a flame detecting system, relay means, a burner, a space discharge device comprising an anode, a cathode, and a control grid, means operatively connecting said device to said relay means so that said relay means is operated only when the current flow between said anode and cathode is above a predetermined value, a source of periodically varying voltage, connections between said anode, said cathode, and said source of voltage including said relay means, a first connection between said grid and said source of voltage at a point operatively negative with respectto the cathode, a second connection between said grid and said source of voltage comprising a conductor leading from said grid to an electrode extending into the pilot burner flame and connections through ground between said pilot burner and said source of power at a point operatively positive with respect to the cathode, a grounded shield surrounding the greater portion of said conductor, and means for preventing a false indication of the presence of.a burner flame by reason of excessive capacitance or leakage at any point between the conductor and the shield.

7. In a flame detecting system, relay means, a burner, a space discharge device comprising an anode, a cathode, and a control grid, means operatively connecting said device to said relay means so that said relay means is operated only when the current flow between said anode and cathode is above a predetermined value, a source of periodically varying voltage, connections between said anode, said cathode, and said source of voltage including said relay means, a first connection between said grid and said source of voltage at a point operatively negative with re spect to the cathode, a second connection between said grid and said source of voltage comprising a conductor leading from said grid to an electrode extending into the burner flame and connections through ground between said burner and said source of power at a point operatively positive with respect to the cathode, a grounded shield surrounding the greater portion of said conductor, and means for preventing a false indication of the presence of a burner flame by reason of excessive capacitance or leakage at any point between the conductor and the shield, said last named means comprising an intermediate shield connected to said source of power at a point operatively negative with respect to the cathode.

8. In a flame detecting system, relay means, a burner, a space discharge device comprising an anode, a cathode, and a control grid, means 013- eratively connecting said device to said relay means so that said relay means is operated only when the current flow between said anode and cathode is above a predetermined value, an electrode extending into the path of the burner flame, and means including a phase shifting bridge comprising a connection between said grid and said electrode operative when said flame is burning to apply to said grid a potential sufliciently close to that of said cathode that sufliicient current flows between said anode and cathode to operate said relay means and operative when said flame is extinguished or when the resistance between said electrode and burner is relatively low, to apply to said grid 9. potential sufliciently negative with respect to said cathode that the relay means is not operated.

9. In a flame detecting system, relay means, a burner, a space discharge device comprising an anode, a cathode, and a control grid, means operatively connecting said device to said relay meansso that said relay means is energized only when the current flow between said anode and cathode is above a predetermined value, an electrode extending into the path of the burner flame, a conductor between said grid and said electrode, a shield surrounding the greater part of said conductor, and means operative when said flame is burning to apply to said grid a potential suiiiciently close to that of said cathode that sutllcient current flows between said anode and cathode to operate said relay means and operative when said flame is extinguished, when the resistance between said electrode and burner is relatively low, or when the impedance between the shield and its enclosed conductor is excessively low, to apply to said grid a potential-sufll ciently negative with respect to said cathode that the relay means is not operated.

10. In a flame detecting system, relay means, a burner, a space discharge device comprising an anode, a cathode, and a control grid, means operatively connecting said device to said relay means so that said relay means is operated only when the current flow between said anode and cathode is above a predetermined value, an electrode extending into thepathof the burner flame, a phase shifting bridge including a center tapped source of power, and a reactance connected directly to one end of said source of power and through the flame to the other end of said source of power, and means for applying between said grid and cathode a voltage determined by the potential difference existing between the center tap of said source of power and the junction of said reactance with said flame, said phase shifting bridge being such that when the flame resistance is normal, the biasing voltage applied to the grid is sufiiciently small that the relay means is operated and that when the flame resistance is either abnormally high or abnormally low, the biasing voltage is sufficiently high that the relay means is not operated.

11. In a burner control system, a main burner, a pilot burner for igniting said main burner, a

grid controlled space discharge device, means for causing a flow of fuel to the burner when the output current of said device is above a predetermined value, a periodically varying source of voltage, means for connecting said voltage source in the output circuit of said device, a phase shifting bridge including a portion of said source of voltage, a reactance, and a conductive path ineluding a gap normally bridged by the pilot burner flame, and means for applying the output voltage of said bridge to the grid whereby the eifective grid voltage varies in accordance with the phase of said bridge output voltage, the constants of said bridge and the connections between said grid and said bridge being such that the voltage between the grid and cathode of said device is sufficiently small to result in a flow of fuel to the burner when the flame resistance is normal but sufliciently large when the flame resistance is either abnormally high or abnormally low to prevent such flow of fuel to the burner.

12. In a burner control system, a main burner, a pilot burner for igniting said main burner, a grid controlled space discharge device, means for causing a flow of fuel to the burner when the output current of said device is above a predetermined value, a periodically varying soiu'ce of voltage, means for connecting said voltage source in the output current of said device, a phasev shifting bridge including-a portion of said source of voltage, a reactance, and a conductive path including a gap normally bridged by the pilot burner flame, means for applying the output voltage of said bridge to the grid whereby the eflective grid voltage varies in accordance with the phase of said bridge output voltage, the constants of said bridge and the connections between said grid and said bridge being such that the voltage between the grid and cathode of said device is sufficiently small to result in a flow of fuel to the burner when the flame resistance is normal but sufliciently large when the flame resistance is either abnormally high or abnormally low to prevent such flow of fuel to the burner, a shield surrounding the connections between the grid and the burner flame, and means operative in the event of an excessive capacitive effect between the shield and the conductor enclosed thereby to increase the voltage between the cathode and grid sufiiciently to prevent a flow of fuel to the burner.

13. In a flame detecting system, relay means, a burner, a space discharge device comprising an anode, a cathode, and a control grid, a periodically varying source of voltage, means for connecting said voltage source in an output circuit including said relay means, said anode, and said cathode, a phase shifting bridge including a portion of said source of voltage, a reactance, and a conductive path including a gap normally bridged by the burner flame, and a grid circuit including in series a portion of said source of voltage and the output voltage of said phase shifting bridge whereby the grid is biased negatively by an amount depending upon the phase relation of the output voltage of said bridge, the

constants of said bridge being such that the grid bias is sufficiently small to cause operation of the relay means when the flame resistance is normal but sufliciently large when the flame resistance is either abnormally high or abnormally low as to prevent operation of the relay means.

14. A flame detecting system for a fuel burner, comp-rising relay means and a circuit for operating said relay means in accordance with the impedance of the burner flame, said circuit, including in combination, a space discharge device having an input circuit and an output circuit, a source of alternating electrical energy, a connection between said output circuit and said relay means including said source of energy, a first connection between said input circuit and said source and effective to prevent the flow of current in said output circuit, and a phase-shifting bridge forming a second connection between said input circuit and said source and including said flame impedance, said phase-shifting bridge being effective when said impedance lies within a predetermined range of values to oppose said first connection and permit a flow of current in said output circuit.

WILLIS H. GILLE.

CERTIFICATE OF CORRECTION. Patent No, 2,251,LL20. February 11, 19in.

WILLIS H. GILLE.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 7,. first column, line 57, claim 1, for "detaching" read detecting'-; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 8th day of April, A. D. 19Lpl.

Henry V an Arsdale, (Seal) Acting Commissioner of Patents,

CERTIFICATE OF CORRECTION; Patent No, 2,25l,L|.20. February ll, 1911.1.

WILLIS H. GILLE.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows Page 7,. first column, line 57, claim 1, for "detaching read -deteoting-; and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 8th day of April, A. D. 191 1- Henry V an Arsdale, (Seal) Acting Commissioner of Patents, 

